Recently, with the remarkable progress in power electronics technology, a power conversion technology in which a DC power or AC power supplied from a DC power source or AC power source is converted into a desired power without substantial power loss using a semiconductor switch or the like has been attracting attention. In particular, in recent years requiring environmental considerations in using the power, it becomes important to efficiently use electrical energy of a storage battery such as a fuel cell, a solar cell, and a secondary cell (hereinafter, referred to as “storage battery or the like”) in addition to electrical energy available from an existing commercial power source.
For this reason, power conversion technology in power electronics field has become indispensable now. A semiconductor switch used in the power conversion apparatus as such converts freely and broadly the power, and performs on-off switching at a high frequency. Thus, in the power conversion, it is required to suppress the switching loss or noise caused by switching of the semiconductor switch.
As a switch for suppressing the switching loss or noise in the power conversion, for example, there is one disclosed in Patent Document 1. Patent Document 1 discloses a bidirectional current switch regenerating snubber energy. In the disclosed switch, four semiconductor switch elements without reverse blocking capability, each having a reverse conducting diode and P-MOSFET connected in parallel, are connected in a full bridge, and a capacitor for absorbing the snubber energy is connected between the upper and lower sides of electric potential. The operation of the bidirectional current switch as such will be described with reference to FIG. 8. FIG. 8 is an explanatory diagram showing a circuit configuration of a conventional bidirectional current switch.
Referring to FIG. 8, a full-bridge circuit is connected between a current terminal 7 and a current terminal 8. The full-bridge circuit is configured by parallel connection of a first series circuit including a semiconductor switch 1A and a semiconductor switch 1B which do not have a reverse blocking capability and are connected in the reverse direction, and a second series circuit including a semiconductor switch 1C and a semiconductor switch 1D which do not have a reverse blocking capability and are connected in the reverse direction.
Further, each of the semiconductor switches 1A to 1D may be constituted by, e.g., a p-channel metal-oxide-semiconductor field-effect transistor (P-MOSFET) and a parasitic diode connected in parallel thereto. Furthermore, a snubber capacitor 4 is provided between midpoints of the first series circuit and second series circuit so as to connect the midpoints thereof. In the first series circuit, a drain electrode Da of the semiconductor switch 1A is connected to a drain electrode Db of the semiconductor switch 1B. In the second series circuit, a source electrode Sc of the semiconductor switch 1C is connected to a source electrode Sd of the semiconductor switch 1D.
Further, a source electrode Sa of the semiconductor switch 1A and a drain electrode Dc of the semiconductor switch 1C are connected to the current terminal 7. In addition, a source electrode Sb of the semiconductor switch 1B and a drain electrode Dd of the semiconductor switch 1D are connected to the current terminal 8. In this bidirectional current switch, gate control signals are respectively applied to gate electrodes Ga to GD of the semiconductor switches 1A to 1D from a control circuit (not shown, hereinafter the same), so that the semiconductor switches 1A to 1D perform an on-off operation in response to the gate control signals applied to the gate electrodes Ga to GD.
First, in the case where the current flows in the forward direction to the current terminal 8 from the current terminal 7, the control circuit sends the gate control signals to the gate electrode Gb of the semiconductor switch 1B and the gate electrode Gc of the semiconductor switch 1C to turn on the semiconductor switch 1B and the semiconductor switch 1C at the same time. At this time, the control circuit does not send the gate control signals to the gate electrode Ga of the semiconductor switch 1A and the gate electrode Gd of the semiconductor switch 1D. However, since the current flows in the forward direction of each parasitic diode by the parasitic diode of each of the semiconductor switch 1A and the semiconductor switch 1D, the current flows through the semiconductor switch 1A and the semiconductor switch 1D. Accordingly, the current flows in the direction from the current terminal 7 to the current terminal 8.
On the other hand, in the case where the current flows in the backward direction to the current terminal 7 from the current terminal 8, the control circuit sends the gate control signals to the gate electrode Ga of the semiconductor switch 1A and the gate electrode Gd of the semiconductor switch 1D to turn on the semiconductor switch 1A and the semiconductor switch 1D at the same time. At this time, the control circuit does not send the gate control signals to the gate electrode Gb of the semiconductor switch 1B and the gate electrode Gc of the semiconductor switch 1C. However, since the current flows in the forward direction of each parasitic diode by the parasitic diode of each of the semiconductor switch 1B and the semiconductor switch 1C, the current flows through the semiconductor switch 1B and the semiconductor switch 1C. Accordingly, the current flows in the direction from the current terminal 8 to the current terminal 7.
Thus, according to the bidirectional current switch, by alternately driving a pair of the semiconductor switches 1A and 1D located on a diagonal line and a pair of the semiconductor switches 1B and 1C located on a diagonal line, the current may flow in both forward and backward directions between the current terminal 7 and the current terminal 8.
Further, when blocking the current between the current terminal 7 and the current terminal 8 in the bidirectional current switch, the control circuit turns off the driven semiconductor switches by stopping the application of the gate control signal to each of the gate electrodes of the semiconductor switches to which the gate control signals have been applied among the semiconductor switches 1A to 1D. Thus, the current that had been flowing during the ON time is diverted to the snubber capacitor 4 and the snubber capacitor 4 is charged until the current becomes zero.
A voltage across the snubber capacitor 4 increases until the current flowing through the snubber capacitor 4 becomes zero, and the current of the bidirectional current switch is blocked automatically by the parasitic diode of the semiconductor switch such that no current flows. Next, when the current is allowed to flow through the bidirectional current switch, for example, when the gate control signals are respectively applied to the gate electrode Ga of the semiconductor switch 1A and the gate electrode Gd of the semiconductor switch 1D by the control circuit, charges that have been charged in the snubber capacitor 4 are discharged through the semiconductor switch 1A and the semiconductor switch 1D, and the energy that has been charged in the snubber capacitor 4 is supplied to the load side.
As an example of the power conversion apparatus using the bidirectional current switch disclosed in Patent Document 1, there is an AC/DC power conversion apparatus disclosed in Patent Document 2. In the AC/DC power conversion apparatus, a capacitor is connected between the DC terminals of four reverse conduction type semiconductor switches (bidirectional current switches) having a single phase full-bridge configuration, and a secondary battery is connected between the DC terminals with a DC inductor therebetween. Further, an AC power source is coupled between AC terminals with an AC inductor therebetween.
Accordingly, a pair of semiconductor switches located on a diagonal line are turned on or turned off synchronously with the phase of a power voltage, so that an AC power source with a frequency lower than the resonance frequency determined by the AC inductor and the capacitor is connected to it. According to the AC/DC power conversion apparatus of Patent Document 2, a large AC inductor is necessary compared to a conventional pulse width modulation (PWM) converter, but the on-off of the reverse conduction type semiconductor switches is performed one time during one cycle of the AC power source in principle, thereby lessening harmonics in the current waveform, and significantly reducing switching loss by reducing the number of on-off operations of the reverse conduction type semiconductor switches.
[Patent Document 1] Japanese Patent Laid-open Publication No. 2000-358359
[Patent Document 2] Japanese Patent Laid-open Publication No. 2008-193817
However, in the AC/DC power conversion device such as disclosed in Patent Document 2, the control circuit turns on and off a pair of reverse conduction type semiconductor switches located on a diagonal line among the reverse conduction type semiconductor switches in synchronization with the voltage phase of the AC power source. In this case, the control circuit sends gate control signals to prevent two pairs located on diagonal lines from being turned on at the same time, and switches the conversion from AC power to DC power and the conversion from DC power to AC power in response to the phases of the gate control signals.
To this end, the control circuit needs to monitor the voltage phase of the AC power source, which may lead to a problem that the control operation of the control circuit becomes complicated. Therefore, there is a demand for a power conversion apparatus which can output a desired power through a simpler control operation. As an example of the power conversion apparatus, particularly, there is a bidirectional DC/DC converter which can bidirectionally convert the DC power to be supplied to the secondary side when the DC power source is connected to the primary side and the storage battery or the like is connected to the secondary side, or the DC power to be supplied to the primary side when the storage battery or the like is connected to the primary side and the DC power source is connected to the secondary side, and it is required to bidirectionally output a desired DC power through a simpler control operation in terms of stable power supply.